UV curable inks (flexo, offset, screen and inkjet) that are used for food packaging applications must fulfill very low migration level. Part of migration originates from the photoinitiator, the co-initiator (also called synergist) and from products of photo cleavage. Benzophenone is the most widely used photoinitiator for ultraviolet (UV) cured overprint varnishes as it has good surface curing, low yellowing and good solubility and is cheap and widely available. However, benzophenone is also known for its relatively strong odor and its ability to migrate and get extracted from print into foodstuffs, even through packaging such as board and plastic wrappers (see EP 1 438 282 B1). This has been partially solved by using “polymeric photoinitiators”, i.e. photoinitiators with higher molecular weight. Most of the polymeric photoinitiators have solubility issues, poor reactivity and have a big impact on ink flow. When used in higher concentration to increase the reactivity, they often act as plasticizer which is detrimental for the mechanical properties of the cured ink.
There is thus a need to develop photo-reactive binders with no or low migration level of the photo-initiator and the synergist, along with a high UV reactivity and with low detrimental effect on ink flow and mechanical properties of the cured ink even when it is used in large concentration.
EP2130817 describes a molecule in which the nitrogen is integrated in the chain with a 1:1 mol ratio between nitrogen and type II photoinitiator. This way there is less flexibility to adapt the nitrogen content to a different stoichiometry. As amines play also a role in preventing oxygen inhibition, it can be advantageous to work with a mole ratio between nitrogen and type II photoinitiator higher than 1.
EP2161290 describes an aromatic amine blended with an acrylated type II photoinitiator. Amine synergists are not covalently bound to the type II photoinitiator compound. The nitrogen and photoinitiator mol % is limited compared with a molecule having both nitrogen and type II photoinitiator covalently bound on the same backbone.